The present invention relates to melt cast compositions containing new energetic copolyurethane thermoplastic elastomers. It concerns mainly the use of melted TNT as a solvent to dissolve these energetic thermoplastic elastomers to produce insensitive, recyclable, high-energy melt cast plastic bonded explosives.
Conventional melt cast explosives consist of TNT or a dispersion of high-energy crystals in TNT. The most simple and widely used formulation is the mixing of melted trinitrotoluene (xe2x80x9cTNTxe2x80x9d) with anti-cracking agent and waxes. Other compositions such as: Composition B, a mixture of TNT and hexahydro-1,3,5-trinitro-1,3,5-triazine (xe2x80x9cRDXxe2x80x9d)(40:60); Octol, a mixture of TNT and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (xe2x80x9cHMXxe2x80x9d)(30:70) and some other TNT compositions are mixed in melt-cast systems. These compositions are usually melted and cast into artillery shells, rockets, bombs etc. where they are allowed to cool and solidify. These explosive formulations exhibit poor mechanical properties and show undesirable defects such as cracks, exudation, voids, brittleness, which can affect the ballistic performance and the impact sensitivity. One way to improve the mechanical properties of the formulations is to introduce a rubbery binder which serves as binding agents for energetic products. These formulations will be able to absorb impact, resist heat, etc. Such munitions are considered to be insensitive or less sensitive than regular munitions.
High-energy solid compositions such as propellants and composite explosives are usually prepared by combining a variety of materials including oxidizers, binders, plasticizers and a curing agent. Many energetic binders are available for use in the preparation of these high-energy compositions. Usually, these binders are obtained by mixing the energetic or non-energetic prepolymers with the other ingredients followed by a curing reaction involving the use of polyisocyanates. The rubbery binders give the insensitive character to high-energy compositions (U.S. Pat. Nos. 5,061,330, 4,985,093, 4,012,245 and 4,988,397). For composite explosives, the use of these binders leads to plastic bonded explosives (xe2x80x9cPBXsxe2x80x9d) which are chemically crosslinked and therefore not recyclable. A disadvantage of this technology resides in the fact that the mixing period is limited by the pot-life due to increased viscosity, and that long curing times at 60-80xc2x0 C. are needed to crosslink the material leading to an expensive and undesirable process.
In general, formulators dislike doing a chemical reaction in their mixes since this leads to more complex batch processes. Moreover, the existing melt cast facilities are not suitable for these cast-cured PBXs.
A better way to prepare PBXs in melt cast facilities is to use thermoplastic elastomers leading to recyclable PBXs. High concentrations of these inert polymers make these compositions less hazardous, but also less energetic. The use of thermoplastic elastomers that are energetic will result in a loss of less energy. Some researchers identified inert thermoplastic elastomers and introduced them in melt cast formulations (U.S. Pat. Nos. 5,949,016, 4,284,442, 4,445,948, 4,978,482 and 4,325,759). Melting and mixing the thermoplastic elastomers with the other ingredients of the formulation to cast the final products was the ultimate goal of this invention. The limitation of this technology is that there are only a few thermoplastic elastomers that melt in the range of 80-100xc2x0 C. and those melting at higher temperatures are not suitable for this process. Energetic thermoplastic elastomers of the type ABA and ABn melting at 83xc2x0 C. were synthesized by Manser and Wardle but were very viscous and difficult to process in melt cast facilities (U.S. Pat. Nos. 4,483,978, 4,707,540, 4,806,613 and 4,952,644). In the present invention, energetic copolyurethane thermoplastic elastomers based on glycidyl azide polymer (xe2x80x9cGAPxe2x80x9d) have been found to be completely dissolved by TNT and were introduced in melt-cast formulations to isolate new insensitive recyclable plastic bonded explosive compositions.
An object of the present invention is to provide a process using melted TNT as a solvent to dissolve energetic copolyurethane thermoplastic elastomers having melting points higher than 100xc2x0 C. This was achieved in melt-cast systems and new insensitive melt-cast recyclable plastic bonded explosive compositions were isolated. This process using a melted explosive as a solvent for thermoplastic elastomers having melting points higher than 100xc2x0 C. is not restricted to TNT and could be extended to other explosives that could eventually replace TNT in melt-cast formulations such as 1,3,3-trinitroazetidine (xe2x80x9cTNAZxe2x80x9d). Plasticizers, energetic or not can also be used to dissolve the copolyurethane and introduce it in the formulation.
In accordance with one aspect of the present invention, there is provided a process to use melted TNT to dissolve energetic copolyurethane thermoplastic elastomer comprising linear polyurethanes physically cross-linked to one another by hydrogen bonds of the formula:
HOxe2x80x94Pxe2x80x94(Dxe2x80x94P)nxe2x80x94Dxe2x80x94Pxe2x80x94OH
wherein D is a group resulting from the reaction of a diisocyanate with hydroxyl groups of two separate prepolymers; P is a dihydroxyl terminated telechelic energetic prepolymer having a functionality of two or less, reacted with the isocyanate groups of two separate diisocyanates and n is 1 to 100. A complete description of the structure and the syntheses of these copolyurethane thermoplastic elastomers can be found in U.S. patent application Ser. No. 09/058,865 (filed April, 1998), which is herein incorporated by reference.
Preferably, the dihydroxyl terminated telechelic energetic polymer has a molecular weight ranging from about 500 to about 10,000 and is selected from the group consisting of glycidyl azide polymer (xe2x80x9cGAPxe2x80x9d), poly 3-nitratomethyl-3-methyloxetane (xe2x80x9cNIMMOxe2x80x9d) and poly glycidyl nitrate (xe2x80x9cGLYNxe2x80x9d), poly 3-azidomethyl-3-methyloxetane (xe2x80x9cAMMOxe2x80x9d) and poly bis 3,3-azidomethyloxetane (xe2x80x9cBAMOxe2x80x9d). These energetic copolyurethane thermoplastic elastomers can include a chain extender. Suitable chain extenders are low molecular weight diols such as ethylene glycol or a diol of the formula:
OHxe2x80x94CH2xe2x80x94(CH2)nxe2x80x94CH2xe2x80x94OH
wherein n is 1 to 8. The chain extenders can have primary hydroxyl or secondary hydroxyl groups. The preferred chain extenders having secondary hydroxyl groups are 2,4-pentanediol or 2,3-butanediol. The chain extenders can also be low molecular weight diamines. Preferably, the diisocyanate is aromatic such as 4,4xe2x80x2 methylenebis-phenyl isocyanate (xe2x80x9cMDIxe2x80x9d) and toluene diisocyanate (xe2x80x9cTDIxe2x80x9d) or aliphatic such as hexamethylene diisocyanate (xe2x80x9cHMDIxe2x80x9d) and isophorone diisocyanate (xe2x80x9cIPDIxe2x80x9d). The resulting copolymer can be dissolved by melted TNT and be mixed with other components of a melt-cast formulation. Cooling of the mixture resulted in the re-formation of the physical crosslinking of the energetic thermoplastic elastomers to yield new recyclable plastic bonded explosives. Aluminum, magnesium and other state of the art additives can be added to the formulation to increase the performance and the mechanical properties.
The advantage of the present invention is to provide a simple way of introducing copolyurethane thermoplastic elastomers having melting points higher than 100xc2x0 C. in melt-cast formulations using the existing facilities. Since melted TNT is used to dissolve the copolyurethane thermoplastic elastomers, the temperature of operation is the same as for the conventional melt cast formulations. Moreover, compared to cast-cure processes, there is no pot-life and no extended curing time to prepare the PBXs. Therefore, this process is an improved way of introducing energetic thermoplastic elastomers having melting points higher than 100xc2x0 C. in melt-cast formulations, leading to new recyclable insensitive melt cast PBXs.
According to one aspect of the present invention, it provides the use of energetic copolyurethane thermoplastic elastomer (xe2x80x9cETPExe2x80x9d) having melting point higher than 100xc2x0 C. of the chemical structure: 
for manufacturing insensitive melt cast plastic bonded explosives.
According to another aspect of the present invention, it provides a method for manufacturing an insensitive plastic bonded explosive (xe2x80x9cPBXxe2x80x9d) comprising dissolving the ETPE of the chemical structure (I) in an explosive solvent in a melt-cast system and cooling at room temperature.
Thermoplastic elastomers typically consist of copolymer chains having monomers A and B distributed throughout the chains as Axe2x80x94Bxe2x80x94A or Axe2x80x94B, where A is the hard segment capable of crystallization or association and gives the thermoplastic behavior to the copolymer, and B is the soft segment which gives the elastomeric behavior to the copolymer. Conventionally, the A segment is formed by a crystalline homopolymer and the B segment is formed by an amorphous homopolymer.
In practice, at room temperature, a thermoplastic elastomer behaves like a rubber because it is cross-linked in the same fashion as a conventional elastomer, but with reversible physical cross-links. Since the physical cross-links are reversible, the thermoplastic elastomer can be melted or dissolved in a solvent, so that the polymer can be mixed with other components of, for example, a melt cast formulation. A gun or rocket propellant or a composite explosive could be isolated upon cooling or evaporating the solvent. Cooling or evaporating the solvent lets the broken physical cross-links reform and the elastomeric properties are recovered. Therefore, obsolete material can be melted or dissolved before the separation of the components, leading to a recyclable material.
In the present invention, a recyclable linear energetic copolyurethane thermoplastic elastomer having the following chemical structure: 
is dissolved by melted TNT. The synthesis of these energetic copolyurethane thermoplastic elastomers was previously described in U.S. patent application Ser. No. 09/058,865.
Generally, the energetic copolyurethane thermoplastic elastomers used in the present invention are obtained by polymerizing a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less with a diisocyanate. Referring to structure (I), the energetic polymer is the elastomeric B segment and the thermoplastic A segment is provided by the group containing the urethane moieties. Suitable hydroxyl terminated energetic polymers are poly-GAP, poly-NIMMO, poly-GLYN, poly-AMMO and poly-BAMO, of molecular weights of about 500 to 10,000. Preferably, the diisocyanate is aromatic such as 4,4xe2x80x2 methylenebis-phenyl isocyanate and toluene diisocyanate, or aliphatic such as hexamethylene diisocyanate and isophorone diisocyanate. Dibutyltin dilaurate is used as the curing reaction catalyst. Chain extenders such as ethylene glycol, 1,3-propanediol, 1,4 butanediol, 2,3-butanediol, 2,4-pentanediol or other low molecular weight diols or diamines may be added to obtain copolyurethanes having different hard segment contents. Likewise, the mechanical properties of the copolyurethane thermoplastic elastomer, which are determined by the numbers of hard and soft segments, can also be adjusted according to required needs.
As shown in structure (I), the urethane groups within the copolymer form hydrogen bonds with the oxygen of another urethane group or with the oxygen of an ether group, resulting in physical cross-links between the chains. The hydrogen bonds between the urethane groups give the hard segments of the thermoplastic elastomer and therefore the thermoplastic behavior. These hydrogen bonds are reversible, and hence, can be broken by dissolving the copolymer in an organic solvent such as melted TNT. Generally, it is possible to break the hydrogen bonds of most thermoplastic elastomers by melting them. However, in the case of GAP-based copolyurethane thermoplastic elastomers, the copolyurethanes should not be melted as both the decomposition of GAP and the melting point of polyurethanes occur at about 200xc2x0 C. According to the literature, when there are enough hard segments to induce crystallinity, the melting points of linear polyurethanes are in the region of 200xc2x0 C. when the thermoplastic content is about 20 to 50% by weight. Before finding that melted TNT can be used as an organic solvent, it was not possible to process these copolyurethanes in melt cast systems. It was postulated that TNT being an electron poor aromatic ring, has a great affinity for the electron rich aromatic rings of the MDI unit allowing the formation of xcfx80-complexes. This could explain why the copolymers wet the crystals so nicely and change dramatically the TNT mechanical properties. The structure and mechanical properties of the copolyurethane thermoplastic elastomers can be varied using different molecular weight of the prepolymers or using chain extenders which result in different hard segment contents. By varying the structure and the concentration (0.5 to 50% weight) of the copolyurethane thermoplastic elastomer in the formulation, a wide variety of energetic insensitive recyclable melt cast PBXs having different mechanical properties can be obtained. Recyclability of these new innovative PBXs was demonstrated showing that all components of the PBX can be quantitatively recuperated.
One limitation of using polymers such as ETPEs in melt cast processes is the fact that they increase the viscosity of the mixes compared to explosives mixes such as Composition B or Octol. High viscosity results in defects such as bubbles in the melt cast formulations and must be avoided. An elegant way to decrease the viscosity of the mix during the processing and casting is to lower the molecular weight of the copolymer or to polymerize the ETPE in-situ. Such in-situ polymerization of components with low molecular weight molecules ensures the lowest viscosity of the melt cast produced from the mixing and casting process.
Accordingly, it is another aspect of this invention to prepare an insensitive melt cast PBX using in-situ polymerization of the ETPE during the mixing and casting of the formulation. To achieve this, TNT or Composition B or Octol is melted and, instead of adding the ETPE as described earlier, the energetic prepolymers, the catalyst and the diisocyanate are added to the formulation followed by a stirring period of one hour. During this time, the copolymerization is taking place and the casting in shells followed by the cooling period that usually stands for hours will ensure that the polymerization is completed. The final result is an insensitive recyclable melt cast PBX with lower viscosity that is easier to process. Further researches will be conducted in this regard.
GAP Mn=2000 was obtained from 3M company, Minnesota, U.S.A. Dibutyltin dilaurate and 4,4xe2x80x2 methylenebis-phenyl isocyanate were obtained from Aldrich Chemical Co., Milwaukee, Wis., U.S.A. Trinitrotoluene was type II TNT (melting point 80.6xc2x0 C.). Octol type 1 (75/25 HMX/TNT) and type II (70/30 HMX/TNT) were obtained from Holston. The Composition B was obtained from Expro Chemical Products. All the copolyurethanes introduced in the melt cast compositions according to the present invention are rubber-like materials obtained following the procedure described in U.S. patent application Ser. No. 09/058,865. In the following examples, three different copolyurethane thermoplastic elastomers are used. They consist of polyurethane based on glycidyl azide prepolymers of molecular weight 1000 g/mole (xe2x80x9cTPE 1000xe2x80x9d), 2000 g/mole (xe2x80x9cTPE 2000xe2x80x9d) and the combination of a prepolymer of 2000 g/mole and a chain extender, resulting in a TPE having 16% w/w of hard segments (xe2x80x9cTPE 16%xe2x80x9d). The resulting polymers were dissolved in melted TNT in a polymer concentration varying from 0.1 to 50% by weight. The resulting solvated materials can be cast and cooled to yield various desirable recyclable PBXs.
Different PBXs were prepared using melted TNT as the solvent with the copolyurethane at varying concentrations and hard segment content to yield PBXs with improved mechanical properties. As an example, TNT itself is a very hard and breakable solid, while the corresponding PBXs with a copolyurethane concentration at 15% by weight can be easily cut with a spatula. Furthermore, by swirling the product between fingers, a ball can be obtained that showed an excellent adhesion to any surface. In addition, other PBXs were also prepared using melted TNT followed by addition of RDX and HMX. Excellent mechanical properties were observed and the resulting performance were comparable to the performance of Composition B. In these formulations, the polymer concentration was set between 10-20%, the TNT concentration was at 20% and the nitramine concentration was at 60%. In general, polymer concentrations and explosive concentrations were set at 5-20% and 80-95% respectively. The sensitivity to impact and friction was decreased in all cases for all PBXs going from 10 N-m and 80 N for pure TNT to 25 N-m and over 360 N for PBXs. The same decrease of sensitivity was observed for Octol (Impact sensitivity 7.5 N-m and friction sensitivity 120 N) and Octol-based PBXs (Impact sensitivity 20 N-m and friction sensitivity over 360 N). All the vacuum stability tests showed stable compounds with an excellent compatibility between the ingredients. The viscosities of the PBXs varied from 200 poise to 800 poise, compared to melt cast Octol (40-150 poise) and cast cured PBX (1000-2000 poise). The thermal analyses demonstrated that introduction of copolyurethane thermoplastic elastomers lowered the melting point of TNT by 0-8xc2x0 C. The glass transition temperatures of the PBXs were in the range of xe2x88x9230xc2x0 C. at a polymer concentration of 20%, the PBXs being not plasticized. The use of plasticizers will lower the glass transition temperature and can also be used to dissolve the copolymers for introduction in the formulation.
From an industrial perspective, instead of using TNT and add RDX and HMX to the formulation, it would be more interesting to use directly available commercial Octol or Composition B explosives. Melting Octol or Composition B means melting TNT and introduction of the copolyurethane at this stage can be easily done. The mixing and casting of these formulations using Octol and Composition B were done and are described in the examples. The concentrations of polymer/Octol or polymer/Composition B were 5-20/80-95% respectively.
This invention is not restricted to the use of TNT as the solvent, other explosives that melt around 80-100xc2x0 C. such as TNAZ could be used. The introduction of plasticizers could also be done to improve the thermal properties at low temperatures and or to dissolve the polymer for their introduction in the formulation. State of the art plasticizers such as DEGDN, TMETN, BDNPA/F or other plasticizers suitable for incorporation with GAP polymers can be added to the formulations. Other additives such as fuel aluminum, magnesium and those generally known in the art can also be included in these new PBXs.
Densities were measured based on standard ASTM D-792-91 method.
The stability of pure compounds and the compatibility with other products were verified by the Vacuum Stability Test based on STANAG 4479 (Explosifs: Essai de stabilitxc3xa9 sous vide, NATO STANAG 4479, First Edition, 1995). One modification has been made on that procedure: the duration of the test was extended to 48 hours at 100xc2x0 C. A gassing of more than 2 ml/g of sample indicates an incompatibility.
Julius Peter impact and friction apparatuses were used to test the sensitivity of mixtures (BICT, Sensitivity to impact: BAM Impact test, test description and procedure, Jun. 25, 1982). The minimum amount of energy necessary to have a sample reacting, either explosion or decomposition, was measured.
The viscosity was evaluated using a Brookfield apparatus model RVF. For each mixture, the measure was taken at four different rates (2, 4, 10 and 20 Rotations Per Minute-RPM).
The detonation velocity of the experimental compositions was measured on cylinders of explosives using ionisation probes. The cylinders were 2.54 cm in diameter and 20 cm long, and they were fired vertically. Initiation was made at the top using a pellet of RDX/wax (98.5/1.5) 3.18 cm in diameter by 2.54 cm long. A length of three diameters was left at the top to stabilise the detonation wave. Three probes were placed, separated by about 5.1 cm from each other, starting at 7.6 cm from the top. A small metal witness plate (7.6 cmxc3x977.6 cmxc3x970.65 cm) was placed underneath the cylinder to confirm the detonation of the cylinder. A small length (2.54 cm) was left at the bottom of the cylinder to avoid reflections off the witness plate. The distance between the probes was measured exactly and a counter recorded the time between each activation of the probes. Two velocities were then recorded. The reported value of the velocity is the total distance between the first and the third probe. The two velocities are used to verify that the velocity is constant and to evaluate the error on the measurement. All the cylinders measured for that study had a constant velocity at a diameter of 2.54 cm.
Thermal analysis were recorded on a Dupont DSC model 2910 under Helium (50 cc/min) between xe2x88x92100xc2x0 C. and 100xc2x0 C. at a rate of 10xc2x0 C./min.
Rifle Bullet tests were performed according to the method found in the Encyclopedia of Explosives and Related Items (Fedoroff, B. T., Sheffield, O. E., Encyclopedia of Explosives and Related Items, PATR 2700, Volume 2, Picatinny Arsenal, Dover, N.J., USA, 1960). The explosives were cast in standard 2-inch pipes that were three inches long and fitted at both ends with standard screwed caps for 2-inch pipes. Bullets of calibre 7.62 mm (Ball C21) were fired at the items.
The following examples illustrate the performance of the process according to the invention.